December 26, 2012

Some Grasses Have An Evolutionary Advantage For Better Photosynthesis

In examining the differences in photosynthetic activity among certain types of grasses, researchers from Brown University found that some plants are positioned to take evolutionary advantage of certain situations.

According to a report in the Proceedings of the National Academy of Sciences, Brown researcher Pascal-Antoine Christin spent two years analyzing the cellular anatomy of 157 living species of two different grass clades, BEP and PACMAD. The team selected these two clades because both have the metabolic framework to become efficiently photosynthetic C4 grasses. However, only species in the PACMAD clade achieve photosynthesis at this higher level, while the rest of the grasses perform it at the lower C3 level.

Using genetic data, the team organized the various species into an evolutionary tree. The organizational structure allowed them to surmise the anatomical traits of the clades´ ancestral grasses, which no longer exist today. This technique, known as ancestral state reconstruction, permitted the evaluation of the evolution of certain anatomical differences over time.

They discovered that in the leaves of many PACMAD C3 grasses, the vascular structures were closer together – and these veins were encircled by larger, “bundle sheath” cells than in BEP C3 grasses. In C4 plants, the PACMAD-style arrangement of cell allows for a more efficient use of carbon dioxide when the gas is in short supply.

"We found that consistently these PACMAD C3s are very different anatomically than the C3 BEPs," said co-author Erika Edwards, an assistant professor of ecology and evolutionary biology at Brown. "We think that was the evolutionary stepping stone to C4-like physiology."

Based on their evolutionary analysis, the researchers found that BEP and PACMAD grasses were similar and evolving in the same direction around 60 million years ago. However, they eventually started to diverge structurally -- with bundle sheath cells surrounding the veins in BEP grasses beginning to shrink down while those in PACMAD grasses remained larger.

"When atmospheric CO2 decreased tens of millions of years after the split of the BEP and PACMAD clades, a combination of shorter [distances between veins] and large [sheath] cells existed only in members of the PACMAD clade, limiting C4 evolution to this lineage," the authors wrote in their paper.

The team noted that some C4 grasses evolved because of useful changes in outer sheath cells, while others saw the upgrade in inner sheath cells.

Edwards said her team´s study enables plant biologists in understanding when and where important plant traits evolved. She said she hopes the findings advance research in basic plant science and also lead to better agricultural studies as well.

"Now that we have this increasingly detailed birds-eye view, we can start to become a more predictive science," she said. "Now we have the raw goods to ask interesting questions about why, for example, one trait evolves 10 times in this region of the tree but never over here. In terms of genetic engineering we're going to be able to provide some useful information to people who want to improve species, such as important crops."

Image 2 (below): The circle-shaped veins are relatively close together in this magnified cross-section of a leaf of Eriachne ciliata grass, and they are ringed by large “bundle sheath” cells. That anatomy promotes a more efficient “C4” means of photosynthesis. Credit: Edwards lab/Brown University